This invention is concerned with an improved iron-based catalyst for synthesizing ammonia from hydrogen and nitrogen.
Iron-based ammonia synthesis catalysts in the oxidized state commonly comprise a reducible iron oxide, usually magnetite (Fe.sub.3 O.sub.4), and lesser amounts of oxides of alkali metals, alkaline earth metals and oxides of Group III metals, which are not readily reducible. Such difficultly reducible oxides are called promoters and include for example potassium oxide, calcium oxide, beryllium oxide, magnesium oxide, lanthanum oxide, silica, thorium oxide, cesium oxide and alumina. Catalysts are doubly promoted if they contain two promoters, commonly K.sub.2 O and Al.sub.2 O.sub.3 ; triply promoted if they contain three promoters, commonly K.sub.2 O, CaO, and Al.sub.2 O.sub.3 ; and more highly promoted for higher combinations. Prior to the catalyst's use in ammonia synthesis it is reduced to convert it to the active state by chemical reduction in a converter wherein the catalyst in the oxide form is contacted with hydrogen or a mixture of hydrogen and nitrogen or ammonia gas. The iron oxide is chemically reduced to iron while the promoters remain virtually unchanged. The promoters, during reduction, reduce the rate of growth of the iron crystals and change the electronic state of iron and its chemisorptive properties for nitrogen.
In the past, acid treatments have been described for leaching out selected portions or constituents of catalysts and catalyst supports. For example, it is known in the art to treat ceramic support materials for catalysts with concentrated acids for the control of the depth of penetration of the catalytically active materials. Similarly, it is known to treat alkali metal-containing alumina support materials with a dilute acid or acid salt to neutralize the alkali metal cations which are not tied-up in the alumina crystallites of the support prior to impregnation with a catalyst such as Pt, Fe, etc. (see U.S. Pat. No. 4,051,072).
In U.S. Pat. No. 4,040,981, a process for producing a denitrating catalyst is described in which the surface of an unactivated metal is plated with aluminum and the resulting layered product is immersed in an aluminum-dissolving solution consisting of an aqueous solution of acid or alkali, e.g. hydrofluoric acid, nitric acid, mineral acids, acetic acid, formic acid and like organic acids to dissolve out the aluminum and activate the surface of the metal.
Further, U.S. Pat. No. 1,926,099 describes the treatment of base exchanged bodies or catalysts with acids such as mineral acids which bodies or catalysts have a nucleus or ion exchangeable portion containing a basic metal oxide, an acid oxide and an exchangeable base portion. The acid treatment removes part or all of the exchangeable bases and also part or all of the basic portion of the nucleus. The basic metals of the nucleus include copper, aluminum, iron and cobalt and the acid portion of the nucleus includes alkali metal silicates and alkali metal salts of acids. The exchangeable bases may be specific catalysts, stabilizers or promoters. Leached non-silicious base exchange bodies, either partially or completely leached, may contain catalytically active components and behave as catalysts, stabilizer promoters or both.
The present invention provides an improved iron-based catalyst for ammonia synthesis formed by leaching a promoted iron based catalyst with a complexing agent under oxidizing conditions to increase the porosity of the pre-leached catalyst. The activity of the catalyst is dramatically improved over the pre-leached catalyst.
In another embodiment of the invention, the leached catalyst is treated with a cobalt complex or salt to incorporate cobalt therein.
In still another embodiment of the invention, the leached catalyst is treated with a nickel salt to incorporate nickel therein. Both the cobalt and nickel incorporated catalysts are useful in ammonia synthesis.